Many water utilities across the U.S. are transitioning to chloramine for disinfection as an alternative to chlorine. This change is in response to stricter U.S. Environmental Protection Agency regulations on disinfection byproducts (DBPs), which are created when chlorine reacts with organics in water. Chloramine, a combination of chlorine and ammonia, is more stable and does not create DPBs.
Removing chloramine at the point of use, however, is more difficult than removing chlorine. Standard granular activated carbon (GAC) and carbon gac products have limited capacity for chloramine reduction. Products known as “catalytic” or “surface-modified” activated carbon can provide a solution.
In general, the catalytic properties of carbon are measured by the rate at which carbon decomposes hydrogen peroxide. The resulting peroxide number, measured in minutes, estimates the carbon’s utility in any catalytic application, including chloramine reduction. Based on the comparative results obtained for different mesh size commercial carbons, the efficiency of chloramine reduction is discussed in the terms of peroxide decomposition capacity and further extended to the total life (volume) claims for corresponding GAC carbon.
Chemistry of Iron Oxidation:
A mineral found in soil, iron normally exists in an insoluble oxide form, namely ferric oxide. If acidic ~ or carbon dioxide ~ containing water passes through the soil, the insoluble ferric oxide is reduced to the very soluble ferrous form. When water is pumped from the ground, oxygen from air enters the water and is available for reaction with the ferrous iron. In the presence of oxygen the ferrous form is eventually oxidized to the insoluble ferric form, resulting in familiar red deposits that stain sinks and clothes.
In iron removal processes, the insoluble ferric hydroxide comes out of solution and is separated from the water by either filtration or settling. Catalytic carbon accelerates the reaction rate of ferrous to ferric iron dramatically, completely removing the in the relatively short time the water is in contact with the carbon
Under normal conditions, the reaction rate of ferrous to ferric iron is fairly slow, even when excess oxygen is present. This slow reaction rate necessitates the use of large retention tank and sedimentation tanks to allow time for precipitation to occur. A separate filtration step is then required to remove the remaining particulate.
In treating iron-laden water, the catalytic properties of the form of granular activated carbon perform quite differently from standard activated carbon. The catalytic properties greatly accelerates the reaction time of iron to an insoluble form. By oxidizing iron from a soluble to less soluble state, catalytic carbon serves to simplify the removal.
The resultant increase in reaction rate that occurs by using catalytic carbon allows smaller pieces of equipment to be used. As with all oxidation techniques, oxygen is required ~ but a simple educ-tor or air injection pump is all that is required. As the reaction occurs, the precipitate is collected on the surface of the carbon, and a secondary filter is not required. Periodic backwash is performed to remove this iron floc and return the carbon to a usable state.
Another benefit of the catalytic carbon is its proven performance in removing hydrogen sulfide (H2S) from water. Many iron-containing waters also contain H2S and same bed of catalytic carbon can be used to remove both.